1. Field of the Invention
The invention relates to a narrow band laser, and particularly to a high power excimer or molecular fluorine laser operating at a high repetition rate (e.g., 1-4 kHz or more).
2. Discussion of the Related Art
Semiconductor manufacturers are currently using deep ultraviolet (DUV) lithography tools based on KrF-excimer laser systems which emit around 248 nm and which will be followed by the next generation of ArF-excimer laser systems operating around 193 nm. Vacuum UV (VUV) lithography uses the molecular fluorine (F2) laser operating around 157 nm.
Fabrication of integrated circuit devices of a quarter-micron or less, requires very fine photolithographic techniques. Bandwidths around 1.0 pm have been deemed sufficient for 248 nm lithography in the past. Today, it is desired to have an excimer or molecular fluorine laser with an output emission bandwidth of less than 0.6 pm and preferably less than 0.4 pm. It is specifically desired to have such a laser today emitting a laser beam with a bandwidth of less than 0.4 pm. Today""s knowledge of derived bandwidth depends on wavelength, and the above is thus less than 0.4 pm for the 248 nm KrF laser, less than 0.3 pm for the 193 nm ArF laser, and less than 0.1 pm for the 157 nm F2 laser.
To produce smaller features on silicon chips using the exposing radiation of a laser operating at the above-mentioned narrower bandwidth, e.g., less than around 0.6 pm, it is desired that the laser system exhibit greater absolute wavelength stability. For example, a laser output beam wavelength stability around or below 0.1 pm is desired.
A narrow band 248 nm lithography laser system operating at low power, e.g., less than 1 kHz, and having a bandwidth around 1 pm or more, includes within its resonator a line-narrowing module, such as that shown by example at FIG. 1 (see also, U.S. Pat. No. 5,150,370, which is hereby incorporated by reference). The line-narrowing module shown includes a prism beam expander including three prisms 30, 30 and 36, an etalon 34 and a grating 38. Each of the prisms 30, 32 and 36, and the plates of the etalon 34 of a line-narrowing module of a conventional KrF laser comprises fused silica due to its transparency at 248 nm. The wavelength stability is, however, substantially above 0.1 pm when the laser is operated at 1 kHz or higher power.
Line-narrowing modules vary in their response to the exposure to high power or high repetition rate laser beams that cause heating and aging of the optical components. For example, nonuniform heating of the optical elements of the line-narrowing module may substantially degrade their quality by, for example, disrupting the planarity of optical surfaces by localized expansion and causing fluctuations in the thermally dependent refractive index, thereby distorting the wavefront of the retroreflected or transmitted beam and detuning the wavelength. Wavefront distortions lead to changes in the output bandwidth of the laser system which is a parameter that it is desired to keep constant. Wavelength detuning may be compensated by rotating an optical element, typically the grating or HR mirror, as mentioned above. In addition to wavefront distortions and detuning, absorption by optical components results in reduced overall efficiency of the laser.
Parameters of the line-narrowing module that depend on the xe2x80x9cqualityxe2x80x9d of the optical components such as the magnitude of angular dispersion, reflectivity for specific wavelengths, linearity (i.e., absence of wavefront distortions), scattering of the beam, etc., will thus affect the bandwidth, linewidth and overall performance of the laser. The quality of the optical components is generally tested by measuring the absorption of DUV (or VUV for F2 lasers) radiation that leads to heating of the optical components and thermally induced distortions and defects.
It is desired to operate the laser at a higher repetition rate than is typical. For example, a conventional xe2x80x9clow powerxe2x80x9d laser might operate at a repetition rate around 600 Hz. It is desired to have a laser operating above 1 kHz, and particularly around 2-4 kHz, or more. For 193 nm lasers, a repetition rate around 4 kHz will be used. To achieve sufficient throughput, the expectation for the molecular fluorine laser is to have a repetition rate greater than 4 kHz. At these repetition rates, greater laser power is incident on surfaces of optical components within the laser resonator that can serve to enhance the effects of thermally induced distortions or defects on the bandwidth of the laser.
It is desired to have a narrow band laser operating at high power and having a wavelength stability at or less than substantially 0.1 pm. In addition to KrF (248 nm) lasers, it is desired to have an efficient line-narrowing module for use with ArF (193 nm) and F2 (157 nm) lasers.
It is therefore an object of the invention to provide a high power laser, i.e., 1-2 kHz or more, exhibiting a narrow bandwidth, such as is provided when a line-narrowing module including a prism beam expander and one or more etalons and/or a grating is included within the laser resonator, preferably less than 0.6 pm.
It is a further object of the invention that such high power narrow-band laser exhibit a wavelength stability of 0.1 pm or less.
The above objects are met by a laser including a gain medium surrounded by a resonator and including a line-narrowing module including a prism beam expander and one or more etalons and/or a grating or grism within the resonator. The material of transmissive portions of the line-narrowing module including the prisms and the plates of the etalons comprises a material having an absorption coefficient of less than 5xc3x9710xe2x88x923/cm at 248 nm incident radiation, less than 10xc3x9710xe2x88x923/cm at 193 nm incident radiation, and less than 0.1/cm at 157 nm. Preferably the material also has a thermal conductivity greater than 2.0 W/mxc2x0 C. Preferably, that material is calcium fluoride (CaF2).